Foam sheet core for composite sandwich structures and method of making the same

ABSTRACT

A foam sheet core, including a plurality of foam sheet walls defining an array of hollow cells, wherein the plurality of foam sheet walls are bonded together to form the array of hollow cells, each of the plurality of foam sheet walls has a thickness from about 0.002 inches to about 0.08 inches, and each of the plurality of foam sheet walls has an average height from about 0.05 inches to about 5 inches.

TECHNICAL FIELD

The present disclosure generally relates to composite sandwichstructures, and more particularly, to foam sheet cores for compositesandwich structures and methods for making the same.

BACKGROUND

Composite sandwich structures generally include a core sandwichedbetween two face sheets. Often, the core includes cells or air pocketsto reduce the weight of the composite sandwich structure as compared toequivalent structures made of solid materials. This reduced weight hasled to increased use of composite sandwich structures in the aerospaceindustry to reduce the weight of aircraft components.

The core may be formed of various materials, including, but not limitedto, metal, laminate, or paper honeycombs, or solid or machined polymerfoams. The face sheets may be formed from composite sheets, such asthose formed of multiple laminated plies of a fiber reinforced resin.However, a number of chemicals are involved in the creation oftraditional honeycombs, such as adhesives and thermoset resins, whichare difficult to recycle. In addition, traditional honeycomb cores havealmost reached a limit in the weight reduction they offer, whiletraditional foam cores lack the large open cells of honeycomb cores.

Accordingly, there is a need for composite sandwich cores that offerimprovements in weight reduction and offer reduction in the useadhesives and thermoset resins, as well as offering variability of coredensity. In addition, there is a desire for composite sandwich coresthat require no curing process or may be formed as an out of autoclaveprocess.

BRIEF SUMMARY

This summary is intended merely to introduce a simplified summary ofsome aspects of one or more implementations of the present disclosure.This summary is not an extensive overview, nor is it intended toidentify key or critical elements of the present teachings, nor todelineate the scope of the disclosure. Rather, its purpose is merely topresent one or more concepts in simplified form as a prelude to thedetailed description below.

The foregoing and/or other aspects and utilities exemplified in thepresent disclosure may be achieved by providing a foam sheet core,including a plurality of foam sheet walls defining an array of hollowcells, wherein the plurality of foam sheet walls can be bonded togetherto form the array of hollow cells, wherein each of the plurality of foamsheet walls can have a thickness from about 0.002 inches to about 0.08inches, and wherein each of the plurality of foam sheet walls can havean average height from about 0.05 inches to about 5 inches.

The plurality of foam sheet walls can be fusion bonded together to formthe array of hollow cells, the foam sheet core can consist essentiallyof the plurality of foam sheet walls defining the array of hollow cells,and the foam sheet core may not include an adhesive.

Each of the array of hollow cells can have an average diameter fromabout 0.1 inches to about 1.0 inches and an average height from about0.05 inches to about 5 inches.

The plurality of foam sheet walls can include a foamed polymer, and thefoamed polymer can include at least one of phenolic, epoxy, polystyrene(PS), polyethylene (PE), polypropylene (PP), polyamide (PA),polyethylene terephthalate (PET), polyether ether ketone (PEEK),polyetherketoneketone (PEKK), polycarbonate (PC), polyetherimide (PEI),polyphenylsulfone (PPSU), thermoplastic polyurethane (TPU),acrylonitrile butadiene styrene (ABS), polyvinyl Chloride (PVC),polyurethane (PU), or combinations thereof.

At least one of the plurality of foam sheet walls can include adifferent foamed polymer than the rest of the plurality of foam sheetwalls.

The foamed polymer can have a relative density to solid from about 2.5%to about 100% and a melting temperature from about 270° F. to about 800°F.

The plurality of foam sheet walls can further include reinforcingfibers, and the reinforcing fibers can include one or more of carbonfibers, fiberglass fibers, aramid fibers, ultra-high molecular weightpolyethylene (UHMWPE) fibers, polyester fibers, polypropylene (PP)fibers, polyethylene (PE) fibers, polyamide fibers, or combinationsthereof.

At least one of the hollow cells in the array of hollow cells caninclude a filling material, and the filling material can include atleast one of rubber foam, epoxy, phenolic, polystyrene (PS) foam,polyurethane (PU) foam, polyester foam, melamine foam, or combinationsthereof.

The filling material can be different from the foamed polymer formingthe foam sheet walls, and the filling material can enhance at least oneof a fire-retardance of the foam sheet core, a rigidity or stiffness ofthe foam sheet core, an acoustic insulation of the foam sheet core, animpact resistance of the foam sheet core, or combinations thereof.

The foregoing and/or other aspects and utilities exemplified in thepresent disclosure may also be achieved by providing a compositesandwich structure including a foam sheet core; and one or more skinpanels, wherein the foam sheet core can be fusion bonded to the one ormore skin panels, and wherein the foam sheet core can include aplurality of foam sheet walls defining an array of hollow cells, whereinthe plurality of foam sheet walls can be bonded together to form thearray of hollow cells, wherein each of the plurality of foam sheet wallscan have a thickness from about 0.002 inches to about 0.08 inches, andwherein each of the plurality of foam sheet walls can have an averageheight from about 0.05 inches to about 5 inches.

The composite sandwich structure can be substantially-free of adhesives.

The one or more skin panels can include one or more of phenolic, epoxy,polypropylene (PP), polyamide (PA), polyethylene terephthalate (PET),polyether ether ketone (PEEK), polyetherketoneketone (PEKK),polycarbonate (PC), polyetherimide (PEI), polyphenylsulfone (PPSU),thermoplastic polyurethane (TPU), acrylonitrile butadiene styrene (ABS),polyvinyl Chloride (PVC), polyurethane (PU), or combinations thereof.

The foregoing and/or other aspects and utilities exemplified in thepresent disclosure may also be achieved by providing a multi-corecomposite sandwich structure, including two or more foam sheet cores;one or more skin panels; and one or more septum, wherein the two or morefoam sheet cores can be fusion bonded to the one or more skin panels andthe one or more septum; and wherein each foam sheet core includes aplurality of foam sheet walls defining an array of hollow cells, whereinthe plurality of foam sheet walls can be bonded together to form thearray of hollow cells, wherein each of the plurality of foam sheet wallscan have a thickness from about 0.002 inches to about 0.08 inches, andwherein each of the plurality of foam sheet walls can have an averageheight from about 0.05 inches to about 5 inches.

The multi-core composite sandwich structure can be substantially-free ofadhesives.

At least one of the two or more foam sheet cores can have a differentfunctional characteristic than the rest of the two or more foam sheetcores.

The foregoing and/or other aspects and utilities exemplified in thepresent disclosure may also be achieved by providing a method of makinga foam sheet core for a composite sandwich structure, including stackingtwo or more foam sheets into a foam sheet block; bonding the stackedfoam sheet block; and slicing the bonded foam sheet block into one ormore strips of foam sheet, wherein the foam sheet core includes aplurality of foam sheet walls defining an array of hollow cells, andwherein the plurality of foam sheet walls can be fusion bonded togetherto form the array of hollow cells.

The method can further include expanding the one or more strips of foamsheet into a foam sheet core, and coating the foam sheet core with aresin coat.

The method can further include filling one or more hollow cells of thearray of hollow cells.

The foregoing and/or other aspects and utilities exemplified in thepresent disclosure may also be achieved by providing a method of makinga composite sandwich structure including providing one or more foamsheet cores; and bonding the one or more foam sheet cores to one or moreskin panels, wherein each of the one or more foam sheet cores includes aplurality of foam sheet walls defining an array of hollow cells.

The method can further include curing the one or more skin panels.

Further areas of applicability will become apparent from the detaileddescription provided hereinafter. It should be understood that thedetailed description and specific examples, while indicating thepreferred embodiment of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in, and constitute apart of this specification, illustrate implementations of the presentteachings and, together with the description, serve to explain theprinciples of the disclosure. In the figures:

FIG. 1 illustrates a foam sheet core according to an implementation ofthe present disclosure.

FIG. 2 illustrates a foam sheet core according to an implementation ofthe present disclosure.

FIG. 3 illustrates a foam sheet core according to an implementation ofthe present disclosure.

FIG. 4 illustrates a composite sandwich structure according to animplementations of the present disclosure.

FIG. 5 illustrates a multi-core composite sandwich structure accordingto an implementation of the present disclosure.

FIG. 6 illustrates a method of making a foam sheet core according to animplementation of the present disclosure.

FIG. 7 illustrates an apparatus for bonding continuous foam sheetsaccording to an implementation.

FIG. 8 illustrates a flow diagram for the method of making a foam sheetcore according to an implementation.

FIG. 9 illustrates a method of making a composite sandwich structureaccording to an implementation of the present disclosure.

FIG. 10 illustrates a flow diagram of aircraft production and servicemethodology.

FIG. 11 illustrates a block diagram of an aircraft.

It should be noted that some details of the figures have been simplifiedand are drawn to facilitate understanding of the present teachingsrather than to maintain strict structural accuracy, detail, and scale.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary implementations of thepresent teachings, examples of which are illustrated in the accompanyingdrawings. Generally, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. Phrases, such as, “in an implementation,” “incertain implementations,” and “in some implementations” as used hereindo not necessarily refer to the same implementation(s), though they may.Furthermore, the phrases “in another implementation” and “in some otherimplementations” as used herein do not necessarily refer to a differentimplementation, although they may. As described below, variousimplementations can be readily combined, without departing from thescope or spirit of the present disclosure.

As used herein, the term “or” is an inclusive operator, and isequivalent to the term “and/or,” unless the context clearly dictatesotherwise. The term “based on” is not exclusive and allows for beingbased on additional factors not described unless the context clearlydictates otherwise. In the specification, the recitation of “at leastone of A, B, and C,” includes implementations containing A, B, or C,multiple examples of A, B, or C, or combinations of A/B, A/C, B/C,A/B/B/BB/C, AB/C, etc. In addition, throughout the specification, themeaning of “a,” “an,” and “the” include plural references. The meaningof “in” includes “in” and “on.” Similarly, implementations of thepresent disclosure may suitably comprise, consist of, or consistessentially of, the elements A, B, C, etc.

It will also be understood that, although the terms first, second, etc.can be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first object, component, orstep could be termed a second object, component, or step, and,similarly, a second object, component, or step could be termed a firstobject, component, or step, without departing from the scope of theinvention. The first object, component, or step, and the second object,component, or step, are both, objects, component, or steps,respectively, but they are not to be considered the same object,component, or step. It will be further understood that the terms“includes,” “including,” “comprises” and/or “comprising,” when used inthis specification, specify the presence of stated features, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, steps, operations,elements, components, and/or groups thereof. Further, as used herein,the term “if” can be construed to mean “when” or “upon” or “in responseto determining” or “in response to detecting,” depending on the context.

All physical properties that are defined hereinafter are measured at 20°to 25° Celsius unless otherwise specified.

When referring to any numerical range of values herein, such ranges areunderstood to include each and every number and/or fraction between thestated range minimum and maximum, as well as the endpoints. For example,a range of 0.5% to 6% would expressly include all intermediate valuesof, for example, 0.6%, 0.7%, and 0.9%, all the way up to and including5.95%, 5.97%, and 5.99%, among many others. The same applies to eachother numerical property and/or elemental range set forth herein, unlessthe context clearly dictates otherwise.

Additionally, all numerical values are “about” or “approximately” theindicated value, and take into account experimental error and variationsthat would be expected by a person having ordinary skill in the art. Itshould be appreciated that all numerical values and ranges disclosedherein are approximate values and ranges. The terms “about” or“substantial” and “substantially” or “approximately,” with reference toamounts or measurement values, are meant that the recitedcharacteristic, parameter, or values need not be achieved exactly.Rather, deviations or variations, including, for example, tolerances,measurement error, measurement accuracy limitations, and other factorsknown to those skilled in the art, may occur in amounts that do notpreclude the effect that the characteristic was intended to provide. Asused herein, “about” is to mean within +/−10% of a stated target value,maximum, or minimum value

Unless otherwise specified, all percentages and amounts expressed hereinand elsewhere in the specification should be understood to refer topercentages by weight. The percentages and amounts given are based onthe active weight of the material. For example, for an active ingredientprovided as a solution, the amounts given are based on the amount of theactive ingredient without the amount of solvent or may be determined byweight loss after evaporation of the solvent.

With regard to procedures, methods, techniques, and workflows that arein accordance with some implementations, some operations in theprocedures, methods, techniques, and workflows disclosed herein can becombined and/or the order of some operations can be changed.

FIG. 1 illustrates a foam sheet core according to an implementation ofthe present disclosure. As illustrated in FIG. 1 , a foam sheet core 100includes a plurality of foam sheet walls 110 defining an array of hollowcells 150.

The array of hollow cells 150 can be formed into different arrayconfigurations. For example, as illustrated in FIG. 1 , the array ofhollow cells 150 can be formed into a hexagonal honeycomb array.

FIGS. 2-3 illustrates foam sheet cores according to implementations ofthe present disclosure. As illustrated in FIGS. 2-3 , the array ofhollow cells 150 can also be formed into a geometric squared-shapedarray or a geometric triangle-shaped array depending on how theplurality of foam sheet walls 110 are bonded together. The shape of thearray of hollow cells 150 can be chosen according to a purpose and/orperformance requirements of the foam sheet core 100.

The plurality of foam sheet walls 110 can be bonded together to form thearray of hollow cells 150. In one implementation, the plurality of foamsheet walls 110 are fusion bonded together to form the array of hollowcells 150. For example, in some implementations, the foam sheet walls110 can be implemented as strips of a foamed polymer 105 bonded togetherto form an array of hollow cells 150.

In one implementation, portions of a foam sheet wall 110 can be fusionbonded to portions of another foam sheet wall 110 to define the array ofhollow cells 150. As used herein, the term “fusion boding” refers to thebonding substrates without any additional intermediate layers, such asan adhesive layer designed to bond substrates. The adhesive layer mayinclude a thermoplastic film layer. For example, two foam sheet walls110 may be fusion bonded by thermal means, and the plurality of foamsheet walls 110 can be thermally fusion bonded together to form thearray of hollow cells 150. For example, the plurality of foam sheetwalls 110 can be thermally fusion bonded at a temperature higher than amelting temperature (Tm) of a foamed polymer 105 of the plurality offoam sheet walls 110.

In some implementations, the plurality of foam sheet walls 110 have arough and/or non-uniform surface configured to enhance a fusion bondingof the plurality of foam sheet walls 110.

In one implementation, the foam sheet core 100 includes a plurality offoam sheet walls 110 defining an array of hollow cells 150, wherein theplurality of foam sheet walls 110 are fusion bonded together to form thearray of hollow cells 150.

As described above, the plurality of foam sheet walls 110 can be bondedwithout the use of adhesives, such as a thermoplastic film layerdesigned to bond substrates. Accordingly, the array of hollow cells 150can therefore not include adhesives. In some implementations, the foamsheet core 100 does not include adhesives. For example, the foam sheetcore 100 can be adhesive-free or the foam sheet core 100 can besubstantially-free of adhesives. In some implementations, the foam sheetcore 100 consists essentially of a plurality of foam sheet walls 110defining an array of hollow cells 150.

In other implementations, for example, where weight concerns arelessened or there is no need to avoid the use of adhesives, theplurality of foam sheet walls 110 can be bonded using adhesives inaddition to or instead of fusion bonding. In some implementations, theuse of adhesives can provide additional reliability and strengthcharacteristics to the bond over fusion bonding alone.

For example, portions of a foam sheet wall 110 can be bonded to portionsof another foam sheet wall 110 using an adhesive, such as athermoplastic film layer designed to bond the foam sheet walls 110, todefine the array of hollow cells 150.

As illustrated in FIG. 1 , each of the hollow cells 150 can have anaverage diameter 113 from about 0.1 inches to about 1.0 inches. Forexample, the hollow cells 150 can have an average diameter 113 fromabout 0.1 inches to about 0.50 inches or from about 0.1 inches to about0.25 inches. In some implementation, the hollow cells 150 have anaverage diameter 113 from about 0.1 inches to about 0.14 inches or fromabout 0.34 inches to about 0.41 inches.

As illustrated in FIG. 1 , the array of hollow cells 150 can have anaverage height 112 along a long axis Z from about 0.05 inches to about 5inches. For example, each of the hollow cells 150 can have an averageheight 112 from about 0.50 inches to about 4 inches, from about 0.50inches to about 3 inches, or from about 0.50 inches to about 2 inches.In some implementations the hollow cells 150 have an average height 112along a long axis Z from about 0.50 inches to about 1.0 inches, fromabout 0.75 inches to about 1.25 inches, or from about 3.75 inches toabout 4.25 inches. Accordingly, each of the plurality of foam sheetwalls 110 can have an average height from about 0.05 inches to about 5inches. For example, each of the plurality of foam sheet walls 110 canhave an average height 112 from about 0.50 inches to about 4 inches,from about 0.50 inches to about 3 inches, or from about 0.50 inches toabout 2 inches.

As illustrated in FIG. 1 , each foam sheet wall 110 can have an averagethickness 111 along a short axis X from about 0.002 inches to about 0.08inches. For example, each foam sheet wall 110 can have an averagethickness 111 from about 0.005 inches to about 0.04 inches or from about0.01 inches to about 0.02 inches. In some implementations, each foamsheet wall 110 can have an average thickness 111 from about 0.0035inches to about 0.0045 inches or from about 0.036 inches to about 0.044inches.

The average diameter, height, and thickness of the hollow cells 150and/or the foam sheet walls 110 forming the array of hollow cells 150,together with the material of the foam sheet walls 110, can determinethe functional and physical characteristics of the resulting foam sheetcore 100. For example, a rigidity or stiffness of a composite sandwichstructure incorporating the foam sheet core 100 varies according to atleast one of a thickness of the plurality of foam sheet walls 110, anaverage diameter of the hollow cells 150, and a material of the foamsheet walls 110. As used herein, the terms “rigidity or stiffness” referto a resistance to deformation in response to an applied force ormoment.

The plurality of foam sheet walls 110 can have a uniform thicknessthroughout the foam sheet core 100. In other implementations, at leastone of the plurality of foam sheet walls 110 can have a differentaverage thickness than an average thickness for the rest of theplurality of foam sheet walls 110. In yet other implementations, atleast a portion of one of the plurality of foam sheet walls 110 can havea different average thickness than an average thickness for the rest ofthe plurality of foam sheet walls 110.

The plurality of foam sheet walls 110 can include a foamed polymer 105.The foamed polymer 105 can be capable of forming thin sheets of foamedpolymer. For example, the foamed polymer 105 can include one or more ofa thermoplastic polymer, a thermoset polymer, an epoxy polymer, orcombinations thereof. In some implementations, the foam sheet walls 110consist essentially of the foamed polymer 105. In other implementations,at least one of the plurality of foam sheet walls 110 consistsessentially of the foamed polymer 105.

According to implementations of the present disclosure, foam sheet walls110 formed of a foamed polymer 105 can have a weight advantage overtraditional paper or laminate honeycomb materials while offering similaror improve functional characteristics, such as stiffness, rigidity,impact resistance, etc.

The foamed polymer 105 can include phenolic, epoxy, polystyrene (PS),polyethylene (PE), polypropylene (PP), polyamide (PA), polyethyleneterephthalate (PET), polyether ether ketone (PEEK),polyetherketoneketone (PEKK), polycarbonate (PC), polyetherimide (PEI),polyphenylsulfone (PPSU), thermoplastic polyurethane (TPU),acrylonitrile butadiene styrene (ABS), polyvinyl Chloride (PVC),polyurethane (PU), or combinations thereof.

For example, the foamed polymer 105 can include at least one of PET, PS,PEI, PPSU, or combinations thereof. In some implementations, the foamedpolymer 105 can include at least one of PET and PS due to the strengthand flammability performance of these materials for aerospaceapplications.

In some implementations, the technical characteristics or properties ofthe plurality of foam sheet walls 110 can vary according to a materialof the plurality of foam sheet walls 110. Accordingly, the plurality offoam sheet walls 110 can include the same foamed polymer 105. In otherimplementations, at least one of the plurality of foam sheet walls 110includes a different foamed polymer 105 than the rest of the pluralityof foam sheet walls 110.

The foamed polymer 105 can have a relative density to solid from about2.5% to about 100%. For example, the foamed polymer 105 can have arelative density from about 20% to about 90%, from about 30% to about80%, or from about 50% to about 60%. Accordingly, the foam sheet walls110 can have a relative density to solid from about 2.5% to about 100%,from about 20% to about 90%, from about 50% to about 60%, or from about30% to about 80%. As used herein, the term “relative density” refers tothe foamed polymer 105 density divided by the corresponding solidpolymer density.

If the density of the foamed polymer 105 is too low, the foam sheet core100 may be too weak and can be easily flattened or crushed. However, ifthe density of the foamed polymer 105 is too high, the foam sheet core100 may tend to break when the foam sheet walls 110 are expanded, asurface of the foam sheet core 100 may be too smooth (or its surfaceenergy too low) resulting in lesser fusion bonding quality. Accordingly,in some implementations, a relative density from about 50% to 60% canprovide a good balance between weight reduction, foam sheet surfaceenergy, and strength.

The foamed polymer 105 can be configured to withstand a curingtemperature for a composite material. For example, the foamed polymer105 can have a melting temperature from about 270° F. to about 800° F.In some implementations, the foamed polymer 105 has a meltingtemperature of 250° F. or greater, 350° F. or greater, or 750° F. orgreater.

The foamed polymer 105 can be configured to withstand a curing pressurefor a composite material. For example, the foamed polymer 105 canwithstand a pressure from about 5 psi to about 300 psi withoutsubstantial deformation.

As illustrated in FIG. 1 , in some implementations, the plurality offoam sheet walls 110 can include reinforcing fibers 120. In general, theplurality of foam sheet walls 110 can include any suitable reinforcingfibers 120. The reinforcing fibers 120 can be used to increase thestrength and stiffness of the plurality of foam sheet walls 110. Thereinforcing fibers can include one or more of carbon fibers, fiberglassfibers, aramid fibers, polyester fibers, hemp fibers, wood fibers, orcombinations thereof. Other fibers that can be used include talc fibers,wollastonite fibers, metal fibers, aromatic polyamide fibers, orcombinations thereof. The plurality of foam sheet walls 110 describedherein can further include reinforcing fibers, and the reinforcingfibers include one or more of carbon fibers, fiberglass fibers, aramidfibers, ultra-high molecular weight polyethylene (UHMWPE) fibers,polyester fibers, polypropylene (PP) fibers, polyethylene (PE) fibers,polyamide fibers, or combinations thereof. For example, the reinforcingfibers 120 can include one or more of carbon fibers, fiberglass fibers,aramid fibers, or combinations thereof. The reinforcing fibers 120 canpotentially improve properties of the resulting foam core 100, such as,shear strength at various directions, leading to stronger and stiffercomponents.

In some implementations, the foam sheet walls 110 consist essentially ofthe foamed polymer 105 and the reinforcing fibers 120. In otherimplementations, at least one of the plurality of foam sheet walls 110consists essentially of the foamed polymer 105 and the reinforcingfibers 120.

As illustrated in FIG. 1 , in some implementations, the foam sheet core100 can include a resin coat 130. In general, the foam sheet core 100can include any suitable resin coat 130. The resin coat 130 can be usedto increase functional characteristics of the foam sheet core 100, suchas flame retardance and stiffness, among other things. As illustrated inFIG. 1 , the resin coat 130 can cover external surfaces of the pluralityof foam sheet walls 110 and may be applied by dipping the foam sheetcore 100 in the resin coat 130. The resin coat 130 can include one ormore of epoxy resin coat, phenolic resin coat, vinylester, polyesterresins, or combinations thereof. However, in some implementations, thefoam sheet core 100 does not include a resin coat 130.

As illustrated in FIG. 1 , the array of hollow cells 150 can include afilling material 170. In general, the filling material 170 is configuredto enhance a functional characteristics of the foam sheet core 100. Forexample, the filling material 170 can enhance at least one of afire-retardance of the foam sheet core 100, a rigidity or stiffness ofthe foam sheet core 100, an acoustic insulation of the foam sheet core100, an impact resistance of the foam sheet core 100, or combinationsthereof. In some implementations, the filling material 170 is the sameas the foamed polymer 105 forming the foam sheet wall 110. In otherimplementations, the filling material 170 is different from the foamedpolymer 105 forming the foam sheet wall 110.

The filling material 170 can include at least one of rubber foam, epoxy,phenolic, polystyrene (PS) foam, polyurethane (PU) foam, polyester foam,melamine foam, or combinations thereof.

In some implementations, at least one of the hollow cells 150 in thearray of hollow cells 150 includes a filling material 170. In otherimplementations, all of the hollow cells 150 in the array of hollowcells 150 include a filling material 170. In some implementations, thefilling material 170 filling the hollow cells 150 is the same for all ofthe hollow cells 150 in the array of hollow cells 150. In otherimplementations, at least one filling material 170 filling a hollow cell150 is different from the rest of the filling material 170 filling theother hollow cells 150 in the array of hollow cells 150. As describedabove, the filling material 170 inside at least one of the hollow cells150 can enhance at least one of a fire-retardance of the foam sheet core100, a rigidity or stiffness of the foam sheet core 100, an acousticinsulation of the foam sheet core 100, an impact resistance of the foamsheet core 100, or combinations thereof.

FIG. 4 illustrates a composite sandwich structure according to animplementations of the present disclosure. FIG. 4 illustrates acomposite sandwich structure that, for instance, could use the foamsheet core 100 described above and illustrated in FIGS. 1-3 . As such,the discussion below will reference various components as illustrated inFIG. 1-3 . As illustrated in FIG. 4 , a composite sandwich structure 300includes a foam sheet core 100 and one or more skin panels 200. The foamsheet core 100 can include a plurality of foam sheet walls 110 definingan array of hollow cells 150, wherein the plurality of foam sheet walls110 are bonded together to form the array of hollow cells 150.

The one or more skin panels 200 are bonded to the foam sheet core 100.For example, the foam sheet core 100 can be fusion bonded to the one ormore skin panels 200. In one implementation, the foam sheet core 100 isthermally fusion bonded to the one or more skin panels 200. In otherembodiment, the foam sheet core 100 is fusion bonded to the one or moreskin panels 200 by the application of heat and/or pressure. For example,by applying a curing temperature and/or pressure to the one or more skinpanels 200 and melting a foamed polymer 105 of the foam sheet core 100enough to fusion bond the foam sheet core 100 to the one or more skinpanels 200. In some implementations, the foamed polymer 105 provides thefoam sheet core 100 a rough and/or non-uniform surface configured toenhance a fusion bonding to the one or more skin panels 200. When anouter skin of the foam sheet core 100 is molten, the underlying cellularlayers can be exposed to heat, resulting in large fusion bondingcontacting surfaces. Accordingly, when surfaces of the foam sheet core100 are fusion bonded, the molten materials can be better mixed andfused together, enhancing fusion bonding quality. In someimplementations, fusion bonding the foam sheet core 100 enhances highspeed fusion bonding, such as high-speed fusion bonding using a roll-fedsystem to feed rolls of foam sheet 10 as described below.

As described above, the foam sheet core 100 can be fusion bonded to theone or more skin panels 200 without the use of adhesives. Accordingly,the composite sandwich structure can therefore not include adhesives oradhesive layers. In some implementations, the composite sandwichstructure 300 is adhesive-free or substantially-free of adhesives. Insome implementations, the composite sandwich structure 300 consistsessentially of the foam sheet core 100 and the one or more skin panels200.

In other implementations, the composite sandwich structure 300 furtherincludes adhesive layers 250, such as a thermoplastic film layer,designed to bond the foam sheet core 100 to the one or more skin panels200.

The one or more skin panels 200 can include one or more of a metal, aceramic, a thermoplastic material, a thermoset material, or combinationsthereof. For example, the one or more skin panels 200 can include one ormore of a thermoplastic material or a thermoset material. The one ormore skin panels 200 can include one or more of phenolic, epoxy,polypropylene (PP), polyamide (PA), polyethylene terephthalate (PET),polyether ether ketone (PEEK), polyetherketoneketone (PEKK),polycarbonate (PC), polyetherimide (PEI), polyphenylsulfone (PPSU),thermoplastic polyurethane (TPU), acrylonitrile butadiene styrene (ABS),polyvinyl Chloride (PVC), polyurethane (PU), or combinations thereof. Insome implementations, at least one of the one or more skin panels 200includes a metal, such as aluminum.

At least one of the one or more skin panels 200 can be an uncuredcomposite skin panel or pre-preg. Carbon fiber plies that have beenimpregnated with an uncured thermoset resin or a thermoplastic resin arereferred to as “pre-preg.” As used herein, the term “pre-preg” refers topre-impregnated composite plies, such as epoxy impregnatedunidirectional composite tape or carbon fiber. A pre-preg may beflexible until it is cured, often by heat and pressure curing or curingwithin an autoclave. Other types of carbon fiber include “dry fiber”which has not been impregnated with thermoset resin but may include atackifier or binder. Dry fiber may be infused with resin prior tocuring. For thermoset resins, the hardening is a one-way processreferred to as curing, while for thermoplastic resins, the resin mayreach a viscous form if it is re-heated. In other implementations, theone or more skin panels 200 can be a cured composite skin panel.

The one or more skin panels 200 can further comprise reinforcing fibers.The reinforcing fibers of the one or more skin panels 200 can includeone or more of carbon fibers, fiberglass fibers, aramid fibers, orcombinations thereof. The reinforcing fibers of the one or more skinpanels 200 can include the reinforcing fibers 120.

FIG. 5 illustrates a multi-core composite sandwich structure accordingto an implementation of the present disclosure. FIG. 5 illustrates amulti-core composite sandwich structure that, for instance, could usethe foam sheet core 100 and/or the composite sandwich structure 300described above and illustrated in FIGS. 1-4 . As such, the discussionbelow will reference various components as illustrated in FIG. 1-4 .

As illustrated in FIG. 5 , a multi-core composite sandwich structure 301can include two or more foam sheet cores 100 and one or more skin panels200. In some implementations, the two or more foam sheet cores 100 canbe separated by one or more intermediate skins or septum 201. Forexample, as illustrated in FIG. 5 , a multi-core composite sandwichstructure 301 includes one or more skin panels 200, a first foam sheetcore 101, a septum 201, and a second foam sheet core 102. The first foamsheet core 101 and the second foam sheet core 102 are implementations ofthe foam sheet core 100 illustrated in FIGS. 1-3 . The septum 201 can bean implementation of the one or more skin panels 200 and include thesame materials as described above. In some implementations, the septum201 can provide structural stiffness, stabilization and/or vibrationdampening to the foam sheet core 100.

In other implementations, the septum 201 can include the foamed polymer105. For example, the septum 201 can be formed of the same materials asthe foam sheet walls 110.

The septum 201 can include one or more of fiber reinforced laminates,thermoset (epoxies, phenolics, etc.), thermoplastic solid sheets,thermoplastic foam sheets, or combinations thereof.

The two or more foam sheet cores 100 can be bonded to the one or moreskin panels 200 and/or the one or more septum 201. The two or more foamsheet cores 100 can be fusion bonded and/or can be bonded usingadhesives. For example, as illustrated in FIG. 4 , the first and secondfoam sheet cores 101 and 102 are bonded to the one or more skin panels200 and the septum 201. The two or more foam sheet cores 100 can befusion bonded. For example, the first and second foam sheet cores 101and 102 can be fusion bonded to the one or more skin panels 200 and theseptum 201. In some implementations, the multi-core composite sandwichstructure 301 is adhesive-free or substantially-free of adhesives. Insome implementations, the multi-core composite sandwich structure 301consists essentially of two or more foam sheet cores 100, one or moreskin panels 200, and one or more septum 201. In other implementations,the multi-core composite sandwich structure 301 further includesadhesive layers 250, such as a thermoplastic film layer, designed tobond the first and second foam sheet cores 101 and 102 to the one ormore skin panels 200 and the septum 201.

The two or more foam sheet cores 100 can be the same or can bedifferent. For example, at least one of the two or more foam sheet cores100 can have a different functional characteristic than the rest of thetwo or more foam sheet cores 100. The functional characteristics of thetwo or more foam sheet cores 100 can be determined by an averagethickness of the foam sheet walls 110 and/or by a filling material 170in the array of hollow cells 150. In some implementations, an averagethickness of the foam sheet walls 110 of one foam sheet core 100 isdifferent from an average thickness of the foam sheet walls 110 for therest of the foam sheet cores 100. In other implementations, a fillingmaterial 170 for at least one of the hollow cells 150 of a foam sheetcore 100 is different from a filling material 170 of a hollow cell 150of the rest of the foam sheet cores 100. For example, first foam sheetcore 101 can have a different functional characteristic than second foamsheet core 102 illustrated in FIG. 5 .

In one implementation, an average thickness of the foam sheet walls 110forming the first foam sheet core 101 is different than an averagethickness of the foam sheet walls 110 forming the second foam sheet core102, and therefore, a rigidity or stiffness of the first foam sheet core101 is different from a rigidity or stiffness of the second foam sheetcore 102. In another implementation, at least a portion of the firstfoam sheet core 101 includes foam sheet walls 110 with a differentaverage thickness than the rest of the first foam sheet core 101 and/oran average thickness of the foam sheet walls 110 forming the second foamsheet core 102. Accordingly, a rigidity or stiffness of said portion ofthe first foam sheet core 101 is different from a rigidity or stiffnessof the second foam sheet core 102.

The first foam sheet core 101 can include a different number or type offilling materials 170 such that the functional characteristics of thefirst foam sheet core 101 is different than the functionalcharacteristics of the second foam sheet core 102. For example, thefilling materials 170 used in the first foam sheet core 101 may beselected to improve a first retardance of the first foam sheet core 101,whereas the filling materials 170 used in the second foam sheet core 102may be selected to improve an impact resistance of the second foam sheetcore 102.

FIG. 6 illustrates a method of making a foam sheet core according to animplementation of the present disclosure. FIG. 7 illustrates anapparatus for bonding continuous foam sheets. FIG. 8 illustrates a flowdiagram for the method illustrated in FIG. 6 . FIGS. 6-8 illustratemethods that, for instance, could be used to make the foam sheet core100 described above. As such, the discussion below will referencevarious components as illustrated in FIG. 1-5 .

As illustrated in FIGS. 6-8 , a method 800 for making a foam sheet core100 begins with stacking two or more foam sheets 10 into a foam sheetblock 20 in operation 810.

The foam sheets 10 can be cut from a roll of foam sheet 1. For example,a roll of foam sheet 1 provides a continuous foam sheet 10 of foamedpolymer 105 that is cut into two or more foam sheets 10 of apredetermined length L. A width of the foam sheets 10 can correspond toa width W of the roll of foam sheet 1.

Stacking two or more foam sheets 10 into a foam sheet block 20 caninclude providing two or more continuous rolls of foam sheet 10 androlling them together to form a foam sheet block 20. The length L of thefoam sheet block 20 can correspond to a thickness of the foam sheet core100 after core expansion. In other implementations, the foam sheet block20 can be sliced along the width W to make foam sheet cores 100 withsmaller thickness.

Operation 820 includes bonding the foam sheet block 20. For example, thetwo or more foam sheets 10 can be bonded as they are sequentiallystacked into the foam sheet block 20. In one implementation, a firstfoam sheet 10 is place in a stack and a second foam sheet 10 is placedover the first foam sheet 10. The second foam sheet 10 is then bonded tothe first foam sheet 10. The second foam sheet 10 can be bonded to thefirst foam sheet 10 only in selected areas. For example, the second foamsheet 10 can be thermally fusion bonded to the first foam sheet 10 usinghot metal stamping. A third foam sheet 10 is then placed on the secondfoam sheet 10 and then at least partially bonded to the second foamsheet 10. As illustrated in FIG. 6 , the bonds 15 are staggered, suchthat, the bonds 15 bonding the third foam sheet 10 and the second foamsheet 10 do not directly overlap the bonds 15 bonding the first foamsheet 10 and the second foam sheet 10. The two or more foam sheets 10can be fusion bonded or can be bonded using an adhesive or adhesivelayer. The bonds 15 between the two or more foam sheets 10 can includesurface areas, lines, or points where the fusion sheets are bonded. Forexample, as illustrated in FIG. 6 , the bonds 15 can include an area ofcontact between surfaces of two contacting foam sheets 10.

In other implementations, the foam sheet block 20 can be formed byfusion bonding two or more foam sheets 10 provided from two or morerolls of foam sheet 1. For example, FIG. 7 illustrates an apparatus forbonding continuous foam sheets 10. As illustrated in FIG. 7 , anapparatus 500 includes two or more rolls of foam sheet 1 to providecontinuous foam sheets 10 of foamed polymer 105 and one or more bonders40. The bonders 40 can use moving blocking plates 55 with open slots tothermally bond two or more continuous foam sheets 10 of foamed polymer105 together. For example, the continuous foam sheets 10 of foamedpolymer 105 can be fusion bonded using hot air with moving blockingplates 55.

In other implementations, the bonders 40 can use moving blocking plates55 (e.g., travelling) with open slots of loop type or reciprocatingtype. As illustrated in FIG. 7 , two or more continuous foam sheets 10of foamed polymer 105 can be fusion bonded with a staggered bonds 15 andcut to a length corresponding to a width of the resulting foam sheetcore 100 to form the foam sheet block 20. The staggered bonds 15 caninclude an area of bonding, a line of bonding, or points of bondingbetween the two or more continuous foam sheets 10 of foamed polymer 105.In some implementations, an average thickness of the foamed polymer 105in one or more of the two or more rolls of foam sheet 1 is differentthan an average thickness for the rest of the two or more rolls of foamsheet 1 to vary a thickness of the resulting foam sheet walls 110 in thefoam sheet core 100, as described above.

Operation 830 includes slicing the bonded foam sheet block 20 into oneor more strips of foam sheet 30. As illustrated in FIG. 6 , after thetwo or more foam sheets 10 are bonded together into a bonded foam sheetblock 20, the bonded foam sheet block 20 can be sliced along a width tocreate one or more strips of foam sheet 30. The bonded foam sheet block20 can be sliced for example, using a bandsaw. In other implementations,the bonded foam sheet block 20 can be sliced for example, using a hotwire cutter.

Operation 840 includes expanding the one or more strips of foam sheet 30into a foam sheet core 100. As illustrated in FIG. 6 , the one or morestrips of foam sheet 30 form a plurality of foam sheet walls 110defining an array of hollow cells 150 when expanded into a foam sheetcore 100.

In some implementations, the method 800 further comprises coating thefoam sheet core 100 with a resin coat 130 in operation 850. The foamsheet core 100 can be dip-coated with the resin coat 130. In otherimplementations, the foam sheet core 100 does not include a resin coat130.

The functional characteristics of the foam sheet core 100 can beadjusted by adding a filling material 170 into one or more of the hollowcells 150 of the array of hollow cells 150. Accordingly, in someimplementations, the method 800 further includes adjusting one or morefunctional characteristics of the foam sheet core 100 by filling one ormore of the hollow cells 150 of the array of hollow cells 150 with afilling material 170 in operation 860. For example, the filling material170 can enhance at least one of a fire-retardance of the foam sheet core100, a rigidity or stiffness of the foam sheet core 100, an impactresistance of the foam sheet core 100, or combinations thereof.

FIG. 9 illustrates a method of making a composite sandwich structureaccording to an implementation of the present disclosure. FIG. 9illustrate a method that, for instance, could be used to make the foamsheet core 100 and the composite sandwich structure 300 described above.As such, the discussion below will reference various components asillustrated in FIG. 1-8 .

As illustrated in FIG. 9 , a method of making a composite sandwichstructure begins with providing one or more foam sheet cores 100 inoperation 910. Each of the one or more foam sheet cores 100 can includea plurality of foam sheet walls 110 defining an array of hollow cells150. In some implementation, the plurality of foam sheet walls 110 arefusion bonded together to form the array of hollow cells 150.

Operation 920 include bonding the one or more foam sheet cores 100 toone or more skin panels 200. The one or more skin panels 200 may includeone or more septum 201. The one or more foam sheet cores 100 can befusion bonded to the one or more skin panels 200. The one or more foamsheet cores 100 can be bonded to the one or more skin panels 200 usingan adhesive.

In some implementations, the one or more skin panels 200 are curedbefore they are bonded to the one or more foam sheet cores 100. In otherimplementations, the one or more skin panels 200 are pre-pregs oruncured before they are bonded to the one or more foam sheet cores 100.

The one or more foam sheet cores 100 can be bonded to the one or moreskin panels 200 during a curing process. Accordingly, the method 900 mayfurther include curing the one or more skin panels 200 in operation 930.In some implementations, applying a curing temperature and/or pressureto the one or more skin panels 200 fusion bonds the one or more foamsheet cores 100 to the one or more skin panels 200. In otherimplementations, the one or more foam sheet cores 100 are already bondedto the one or more skin panels 200 before a curing temperature and/orpressure is applied.

Implementations of the present disclosure may find use in a variety ofpotential applications, particularly in the transportation industry,including for example, aerospace, marine, rail, automotive applications,and other application where composite sandwich structures are desired.However, the present disclosure is not limited thereto, andimplementations of the present disclosure may be used in applicationsoutside the transportation industry. Thus, referring now to FIGS. 10 and11 , implementations of the disclosure may be used in the context of anaircraft manufacturing and service method 1000 as shown in FIG. 10 andan aircraft 2000 as shown in FIG. 11 . While FIG. 11 is described interms of an aircraft 2000, the present disclosure is not limitedthereto, and the service method 1000 can be applied to other structures.During pre-production, exemplary method 1000 may include specificationand design 1102 of the aircraft 2000 and material procurement 1104.During production, component and subassembly manufacturing 1106 andsystem integration 1108 of the aircraft 2000 takes place. Thereafter,the aircraft 2000 may go through certification and delivery 1110 inorder to be placed in service 1112. While in service by a customer, theaircraft 2000 is scheduled for routine maintenance and service 1114,which may also include modification, reconfiguration, refurbishment, andso on.

Each of the processes of method 1000 may be performed or carried out bya system integrator, a third party, and/or an operator (e.g., acustomer). For the purposes of this description, a system integrator mayinclude without limitation any number of aircraft manufacturers andmajor-system subcontractors; a third party may include withoutlimitation any number of vendors, subcontractors, and suppliers; and anoperator may be an airline, leasing company, military entity, serviceorganization, and so on.

As shown in FIG. 11 , the aircraft 2000 produced by exemplary method1000 may include an airframe 2115 with a plurality of systems 2118 andan interior 2120. Examples of systems 2118 include one or more of apropulsion system 2122, an electrical system 2124, a hydraulic system2126, and an environmental system 2128. Any number of other systems maybe included. Although an aerospace example is shown, the principles ofthe disclosure may be applied to other industries, such as the marineand automotive industries.

Systems and methods exemplified herein may be employed during any one ormore of the stages of the aircraft manufacturing and service method1000. For example, components or subassemblies corresponding tocomponent and subassembly manufacturing 1106 may be fabricated ormanufactured in a manner similar to components or subassemblies producedwhile the aircraft 2000 is in service. Also, one or more apparatusexamples, method examples, or a combination thereof may be utilizedduring the component and subassembly manufacturing 1106 and systemintegration 1108, for example, by substantially expediting assembly ofor reducing the cost of an aircraft 2000. Similarly, one or more ofapparatus examples, method examples, or a combination thereof may beutilized while the aircraft 2000 is in service, for example and withoutlimitation, to maintenance and service 1114.

While FIGS. 10 and 11 describe the disclosure with respect to aircraftand aircraft manufacturing and servicing, the present disclosure is notlimited thereto. The systems and methods of the present disclosure mayalso be used for spacecraft, satellites, rotorcraft, submarines, surfaceships, automobiles, autonomous vehicles, tanks, trucks, power plants,railway cars, and any other suitable type of objects.

The present disclosure has been described with reference to exemplaryimplementations. Although a few implementations have been shown anddescribed, it will be appreciated by those skilled in the art thatchanges can be made in these implementations without departing from theprinciples and spirit of preceding detailed description. It is intendedthat the present disclosure be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

What is claimed, is:
 1. A foam sheet core, comprising: a plurality offoam sheet walls defining an array of hollow cells, wherein theplurality of foam sheet walls are bonded together to form the array ofhollow cells, wherein each of the plurality of foam sheet walls has athickness from about 0.002 inches to about 0.08 inches, and wherein eachof the plurality of foam sheet walls has an average height from about0.05 inches to about 5 inches.
 2. The foam sheet core of claim 1,wherein the plurality of foam sheet walls are fusion bonded together toform the array of hollow cells, wherein the foam sheet core consistsessentially of the plurality of foam sheet walls defining the array ofhollow cells, and wherein the foam sheet core does not include anadhesive.
 3. The foam sheet core of claim 1, wherein each of the arrayof hollow cells has an average diameter from about 0.1 inches to about1.0 inches.
 4. The foam sheet core of claim 1, wherein the plurality offoam sheet walls comprise a foamed polymer, and wherein the foamedpolymer comprises at least one of phenolic, epoxy, polystyrene (PS),polyethylene (PE), polypropylene (PP), polyamide (PA), polyethyleneterephthalate (PET), polyether ether ketone (PEEK),polyetherketoneketone (PEKK), polycarbonate (PC), polyetherimide (PEI),polyphenylsulfone (PPSU), thermoplastic polyurethane (TPU),acrylonitrile butadiene styrene (ABS), polyvinyl Chloride (PVC),polyurethane (PU), or combinations thereof.
 5. The foam sheet core ofclaim 4, wherein at least one of the plurality of foam sheet wallscomprises a different foamed polymer than the rest of the plurality offoam sheet walls.
 6. The foam sheet core of claim 4, wherein the foamedpolymer has a relative density to solid from about 2.5% to about 100%and a melting temperature from about 270° F. to about 800° F.
 7. Thefoam sheet core of claim 4, wherein the plurality of foam sheet wallsfurther comprise reinforcing fibers, and wherein the reinforcing fiberscomprise one or more of carbon fibers, fiberglass fibers, aramid fibers,ultra-high molecular weight polyethylene (UHMWPE) fibers, polyesterfibers, polypropylene (PP) fibers, polyethylene (PE) fibers, polyamidefibers, or combinations thereof.
 8. The foam sheet core of claim 4,wherein at least one of the hollow cells in the array of hollow cellscomprises a filling material, and wherein the filling material comprisesat least one of rubber foam, epoxy, phenolic, polystyrene (PS) foam,polyurethane (PU) foam, polyester foam, melamine foam, or combinationsthereof.
 9. The foam sheet core of claim 8, wherein the filling materialis different from the foamed polymer forming the foam sheet walls, andwherein the filling material enhances at least one of a fire-retardanceof the foam sheet core, a rigidity or stiffness of the foam sheet core,an acoustic insulation of the foam sheet core, an impact resistance ofthe foam sheet core, or combinations thereof.
 10. A composite sandwichstructure comprising: a foam sheet core; and one or more skin panels,wherein the foam sheet core is fusion bonded to the one or more skinpanels, and wherein the foam sheet core comprises a plurality of foamsheet walls defining an array of hollow cells, wherein the plurality offoam sheet walls are bonded together to form the array of hollow cells,wherein each of the plurality of foam sheet walls has a thickness fromabout 0.002 inches to about 0.08 inches, and wherein each of theplurality of foam sheet walls has an average height from about 0.05inches to about 5 inches.
 11. The composite sandwich structure of claim10, wherein the composite sandwich structure is substantially-free ofadhesives.
 12. The composite sandwich structure of claim 10, wherein theone or more skin panels comprises one or more of phenolic, epoxy,polypropylene (PP), polyamide (PA), polyethylene terephthalate (PET),polyether ether ketone (PEEK), polyetherketoneketone (PEKK),polycarbonate (PC), polyetherimide (PEI), polyphenylsulfone (PPSU),thermoplastic polyurethane (TPU), acrylonitrile butadiene styrene (ABS),polyvinyl Chloride (PVC), polyurethane (PU), or combinations thereof.13. A multi-core composite sandwich structure, comprising: two or morefoam sheet cores; one or more skin panels; and one or more septum,wherein the two or more foam sheet cores are fusion bonded to the one ormore skin panels and the one or more septum; and wherein each foam sheetcore comprises: a plurality of foam sheet walls defining an array ofhollow cells, wherein the plurality of foam sheet walls are bondedtogether to form the array of hollow cells, wherein each of theplurality of foam sheet walls has a thickness from about 0.002 inches toabout 0.08 inches, and wherein each of the plurality of foam sheet wallshas an average height from about 0.05 inches to about 5 inches.
 14. Themulti-core composite sandwich structure of claim 13, wherein themulti-core composite sandwich structure is substantially-free ofadhesives.
 15. The multi-core composite sandwich structure of claim 13,wherein at least one of the two or more foam sheet cores has a differentfunctional characteristic than the rest of the two or more foam sheetcores.
 16. A method of making a foam sheet core for a composite sandwichstructure, comprising: stacking two or more foam sheets into a foamsheet block; bonding the stacked foam sheet block; and slicing thebonded foam sheet block into one or more strips of foam sheet, whereinthe foam sheet core comprises a plurality of foam sheet walls definingan array of hollow cells, and wherein the plurality of foam sheet wallsare fusion bonded together to form the array of hollow cells.
 17. Themethod of claim 16, further comprising: expanding the one or more stripsof foam sheet into a foam sheet core, and coating the foam sheet corewith a resin coat.
 18. The method of claim 16, further comprising:filling one or more hollow cells of the array of hollow cells.
 19. Amethod of making a composite sandwich structure comprising: providingone or more foam sheet cores; and bonding the one or more foam sheetcores to one or more skin panels, wherein each of the one or more foamsheet cores comprises a plurality of foam sheet walls defining an arrayof hollow cells.
 20. The method of claim 19, further comprising: curingthe one or more skin panels.